Papers by Author: Jong Seo Park

Abstract: A nondestructive rod compression has been proposed as a new strength characterizing technique for in-service components. Using electrical discharge machining, milling machining, and stamping, it was possible to machine small rods, typically about 0.5 mm in both diameter and height on the target surface. Static compressions of the rods were carried out using a flat punch and their deformation behaviors were recorded as load-displacement curves. Referring to initial dimension of the rods, engineering stress-strain curves implying the yield strengths were calculated from the deformation curves. Surface yield strengths from the rod compressions were directly compared with reference compression results and their slight discrepancies were discussed from the influences of pre-deformation and damage of the rod and additive compliance of the base.

Abstract: In spite of frequent defect in industrial boilers, life assessment or diagnostic method for them has not been studied. In this research, SB410 carbon steel used in industrial boilers is simulated with artificial aging heat treatment. To do qualitative life assessment, differences in micro-structures and hardness of SB410 by the degradation time are studied. In addition, variation in material properties by aging was observed with the tensile test at room temperature and 179 °C and changes in ductile to brittle transition temperature was observed with the charpy impact test performed at several test temperature.

Abstract: The nondestructive evaluation technique for the material degradation is necessary because of the limitation of conventional destructive methods. In this study, an ultrasonic velocity measurement method was attempted for the estimation of the creep damage of degraded modified 9Cr-1Mo steel. The specimens with seven different kinds of aging periods were prepared by an isothermal heat treatment at 690
. The ultrasonic velocity was measured by an immersion method. The correlation between the measured ultrasonic velocity and tensile properties were studied. The ultrasonic velocity has an declining tendency with the increase of heat treatment time. A correlation between the ultrasonic velocity and aging parameter were established, which allows one to estimate the material degradation of modified 9Cr-1Mo steel.

Abstract: Many researchers have been interested in the nondestructive measurement methods for examining the microstructural changes and components damage in order to assure the safe operation of steel structure. It has been recognized that the techniques based on magnetic measurement offered a great potential because of high susceptibility to the change of several metallurgical factors. In this study, the effect of isothermal heat treatments, which simulate the microstructural changes observed in reactor vessel material at the service temperature, on the magnetic properties was investigated. 2.25Cr-1Mo steel specimens with several different kinds of aging were prepared by an isothermal heat treatment at three different temperature levels. Magnetic property such as coercive force was measured. The coercive force at room temperature monotonously increased with the extent of degradation of the material. The correlation between the measured magnetic property and the mechanical properties was studied. In addition, the applicability of magnetic properties measurements to the evaluation for toughness degradation of reactor vessel was discussed.

Abstract: Several nondestructive evaluation methods were attempted for the estimation of the creep damage of degraded 2.25Cr-1Mo steel. The specimens of three different aging periods were prepared by an isothermal heat treatment at 430°C, 482°C, and 515°C. The effect of probe configuration on the electrical resistivity was studied. Single configuration method and dual-configuration method were utilized for measuring electrical resistivity. The electrical resistivity was determined by a standard DC four-point probe method at 24±0.5°C. Indentation test, magnetic characteristics test and ultrasonic test were carried out to investigate the correlation between the major characteristics and aging parameter. Unlike the electrical resistivity characteristics, ultrasonic attenuation coefficient and indentation characteristics did not show a relation to Larson-Miller parameter. However, a correlation between the electrical resistivity and aging parameter was identified, which allows one to estimate the extent of material degradation.

Abstract: The high cycle fatigue properties of two kinds of wrought Al 2519 alloys without and with scandium of 0.10% were investigated. The fatigue strength was determined at R = 0.1 under constant amplitude loading conditions in air. The alloy with scandium of 0.10% showed a little lower tensile yield strength and higher fatigue strength values. The fine grained Al-0.10Sc alloy exhibited a higher resistance against fatigue crack nucleation despite the lower yield stress in comparison to the coarse grained Al 2519 alloy. The results can be explained mainly with the microstructural differences between both alloys. This results are due to the presence of coherent Al3 (Sc, Zr) precipitates and a very fine subgrain structure.